The resolution of an image provided by an ordinary optical microscope is limited by diffraction limit (i.e., wavelength of light).
On the contrary, an optical image having a resolution exceeding the wavelength of light may be provided by using a near-field optical microscope having a probe of a nanometer-size structure. In addition, by utilizing this near-field optical microscope technique, measurement of shape and spectrum, memory operation (write/read/erase), and optical processing of objects, such as, a biological sample, a semiconductor sample, an optical memory material and a photosensitive material, may be carried out with a resolution of the order of nanometer.
An example of the near-field optical microscope is shown in FIG. 1. This near-field optical microscope 1 is adapted for detecting an evanescent light localized in a region which is extremely close to the surface of an object at a distance smaller than the wavelength of light, so as to measure the shape of the object.
Specifically, an evanescent light 2.alpha. which is generated by irradiating an object 3 with a laser beam 2 under the condition of total internal reflection is scattered at the distal end of a nanometer-size tapered portion 5 of a probe 4. In the near-field optical microscope 1 shown in FIG. 1, the probe 4 is made of an optical fiber, and the light scattered by the tapered portion 5 of the optical fiber probe 4 is guided to a core of the optical fiber through the tapered portion 5. The light guided to the core is propagated within the core, then radiated from the other end (radiation end) of the optical fiber, and detected by a detector. In this case, a two-dimensional image of detection light may be provided by causing the optical fiber probe 4 to scan on the object 3.
As the optical fiber probe 4 used in the near-field optical microscope 1, an optical fiber probe on which a light-shielding coating layer 6 made of a metal or the like is formed except for the distal end of the tapered portion 5 as shown in FIG. 2 (i.e., so-called apertured probe) may be used as well as an optical fiber probe having a conical tapered portion with its distal end formed in nanometer size (i.e., so-called probe tip). In the apertured probe, the scattered light is not transmitted through a portion where the light-shielding coating layer 6 is formed, and the scattered light is transmitted only at the distal end where the light-shielding coating layer 6 is not formed. That is, the fiber having the distal end exposed from an aperture portion 6a of the light-shielding coating layer 6 covering the tapered portion 5 is used as the optical fiber probe 4.
The near-field optical microscope as described above is adapted for collecting the evanescent light generated on the object by using the probe, and its mode is referred to as a collection mode.
As other modes of the near-field optical microscope, there have been known an illumination mode for providing an optical image by locally illuminating an object with an evanescent light generated at the distal end of the probe, and an illumination/collection mode for locally illuminating an object with an evanescent light generated at the distal end of the probe while detecting the light scattered at the distal end of the probe through the probe.
Meanwhile, the phenomenon of energy transfer between the object and the probe in the near-field optical microscope as described above is based on a short range interaction between the dipoles thereof. The conditions for generating an effective interaction between the object and the probe include: first, that the size of the object and the size of the probe are proximate to each other; and second, that the distance between the object and the probe is equal to or smaller than the size of the probe. The size of the probe in this case means the distal end diameter in the probe tip and the aperture diameter in the apertured probe. Therefore, the maximum resolution of the near-field optical microscope is determined by the distal end diameter or the aperture diameter of the probe.
Up to now, a technique for producing the probe tip having the conical tapered portion 5 by tapering one end of the optical fiber by chemical etching and then forming the light-shielding coating layer 6 except for the distal end of the tapered portion 5 by a vacuum evaporation method has been employed as an effective method for producing the apertured probe.
When the taper angle .theta. is small, the light is significantly absorbed in the metal in a region where the cross-sectional diameter of the tapered portion 5 is equal to or smaller than the wavelength, and the transmission efficiency is lowered. (It is to be noted that the dimension from the distal end portion to the position where the cross-sectional diameter equals to the wavelength .lambda. of the light is hereinafter referred to as a tip length L.)
However, if the taper angle .theta. is increased (i.e., the tip length L is decreased) to enhance the transmission efficiency, the light leaks from a thin metal portion on the periphery of the aperture. Therefore, it is difficult to obtain an image with a high spatial resolution equivalent to that in the case of the small taper angle.
The light-shielding coating layer 6 provided on the surface of the tapered portion 5 is conventionally formed by a dry film forming method, such as, vacuum evaporation. However, if the light-shielding coating layer 6 of aluminum (Al) having a thickness of 120 nm is formed by vacuum evaporation, the distal end of the optical fiber is not exposed from the aluminum and is covered with the aluminum film having a thickness of 30 nm. Also, since a vacuum evaporation unit having an optical fiber rotation mechanism and a high degree of vacuum are required for carrying out vacuum evaporation, a significant reduction in production cost due to mass production of the optical fiber probes cannot be expected.
Moreover, it is known that a metal tip or a metallized dielectric tip having a thin metal film of 1 to 50 nm (of a so-called plasmon probe) has a high scattering efficiency because of the large dielectric constant of its metal, and that a strong near field is generated at the distal end of the tip on the basis of the near field enhancement effect of the plasmon excited on the metal surface at the time of optical irradiation. However, it is difficult to use such probe in the illumination mode because of its insufficient light shielding ability.
Meanwhile, the near-field optical microscope using a light as a medium is capable of carrying out local measurement of the wavelength spectrum as well as measurement of the shape of the sample.
For example, in the case where near-field optical spectroscopic study of a semiconductor device is to be carried out, it is simple and effective to employ the illumination/collection mode for performing both optical irradiation and optical detection with the optical fiber probe. However, since the conventional optical fiber probe has a low transmittance, a low illumination efficiency in illumination (or lighting), and a low collection efficiency in collection (or light condensation), actual measurement is difficult with the conventional optical fiber probe. In addition, since the conventional optical fiber probe has the core made of quartz (SiO.sub.2) doped with germanium dioxide (GeO.sub.2) having a low transmittance of ultraviolet rays because of absorption at 350 nm, the conventional optical fiber probe cannot be used for ultraviolet rays.
Thus, in view of the foregoing status of the art, it is an object of the present invention to provide an optical fiber probe which ensures a sufficient light transmission efficiency and enables miniaturization of the aperture diameter, and a manufacturing method therefor.
It is another object of the present invention to provide an optical fiber probe which enables easy formation of a light-shielding coating layer having a minute aperture in a tapered portion of an optical fiber and has an excellent resolution, and a manufacturing method therefor which enables improvement in productivity.
It is still another object of the present invention to provide an optical fiber probe which is excellent in both resolution capability and scattering efficiency, and a manufacturing method therefor.
It is still another object of the present invention to provide an optical fiber probe having excellent transmittance of ultraviolet rays, and a manufacturing method therefor.
It is a further object of the present invention to provide an optical fiber probe which enables propagation of both an excitation light and a detection light generated from a sample, and a manufacturing method therefor.